Roughly, I don't really keep track of precise amounts:
1 cup of water, 1 cup sugar, 1 tbsp salt, a few sprigs of lavender (dried lavender works almost as well). Bring to boil and simmer for 15min to form a simple syrup. This will keep in the fridge for about two weeks. This simple syrup is a pretty fantastic ingredient overall.
40:30:10:20 mango juice:kombucha:grape fruit:soda. Simple syrup: 1.5 fl oz (shot/tot glass) per glass is a good starting point. Something acidic like yuzu vinegar (or whatever clear vinegar you have on hand): a few tbsp. In a tall glass with ice.
I use flavored kombucha, I have found Trilogy by Synergy (available basically everywhere in US) works well with this recipe - but experiment with whatever you have.